Process to generate heat

ABSTRACT

The invention relates to a process to generate heat by burning a liquid fuel in an evaporator burner oven, wherein the liquid fuel is a Fischer-Tropsch derived fuel. The fuel boils for more than 90 wt % between 160 and 400° C. and has a Fischer-Tropsch product which contains more than 80 wt % of iso and normal paraffins, less than 1 wt % aromatics, less than 5 ppm sulfur and less than 1 ppm nitrogen and wherein the density of the Fischer-Tropsch derived product is between 0.65 0.8 g/cm 3  and 0.8 g/cm 3  at 15° C.

The invention is directed to a process to generate heat by burning aliquid fuel in an evaporator burner oven.

Processes to generate heat in domestic applications are known, whereinkerosene or gasoil is used as fuel in evaporator burner ovens. Examplesof such ovens are supplied by Jotul ASA Norway, AGA Foodservices Group,Sunpot, and Corona plc. The ovens are technically simple and oftenrequire no additional moving parts to operate. For example the fuel maybe supplied to the oven by means of gravity wherein the fuel tank ispositioned at a somewhat elevated position relative to the oven itself.If the tank is empty the user will typically have to refill the tank byhand. This method of generating heat, for example to provide domesticheating, lighting or household cooking, is very popular in regions whichare not provided with a natural gas supply means. The fuel most oftenused is kerosene.

A disadvantage of the use of such ovens is that they sometimes fail tofunction due to coke deposits inside the oven. Coke deposits may form atthe bottom plate of the burner when operating at a low power demand.

Especially when such ovens are used for domestic heating during long andstrong winters such breakdown is not favored.

The object of the present invention is to provide a process wherein thereliability and efficiency of the evaporator burner oven is improved.Additionally, emissions are reduced and health and safety issues aretherefore improved.

This object is achieved with the following process. Process to generateheat by burning a liquid fuel in an evaporator burner oven, wherein theliquid fuel comprises a Fischer-Tropsch derived fuel.

Applicants found that when a Fischer-Tropsch derived fuel is used lesscarbon deposits tend to form. It is found that the Smoke Number, whichis correlated with the amount of carbon deposits, is significantly lowerwhen a Fischer-Tropsch derived fuel is used. Because of the lower carbondeposits less failure of the oven will result. Furthermore a decrease insoot deposits will also be beneficial for achieving a better heattransfer, thereby maintaining a high efficiency of the oven over aprolonged period of time. An additional advantage is that this fuel hasno significant odour. The traditionally used kerosene fuel normally hasa strong smell and any spills of kerosene on clothing and ground whilefilling the tank will be smelled for a prolonged time. By using theFischer-Tropsch derived fuel a much more consumer friendly process isobtained. Applicants have further found that the carbon monoxideemissions and the unburned hydrocarbon emissions are significantly lowerwhen using the Fischer-Tropsch derived fuel when compared to thetraditional kerosene fuel.

A further advantage is that this process is an attractive alternative towood burning, which fuel is still often used for household cooking inless developed regions around the world.

Finally the Fischer-Tropsch derived fuel is biodegradable. Thus anyspills or leaking tank vessels will not effect the environment as wouldbe the situation when a petroleum derived kerosene is used.

The evaporator burner oven, which may be used in the process of thepresent invention, may be any oven known to one skilled in the art,which operates, by combustion of evaporating liquid fuel with an oxygencontaining gas. In such ovens the fuel is supplied to a surface whereinit evaporates into a space surrounding said surface and wherein theevaporated fuel is combusted with oxygen containing gas supplied to saidspace. Such a surface may be a wick or the exterior of fuel supplyconduits which conduits are provided with openings to discharge saidfuel from the interior to said exterior surface. Such evaporating burnerovens are for example described in general textbook“Heizung+Klimatechnik 01/02” German Version by Recknagel, Sprenger,Schramek, ISBN: 3-468-26450-8 on page 718. Examples of such evaporatorburner ovens are the so-called Forced Air Type Open Oil Heater, NaturalDraft Open Wick Type Oil Heater, the ovens as manufactured by Jotul fromNorway, as for example the Jotul 709 Oven, the well known AGA cooker asmanufactured by the Aga Foodservice Group plc and similar ovens of othersuppliers such as for example Windhager, Schraak, Haas & Sohn, Buderus,Sunpot or Corona.

The evaporating burner oven should be distinguished from burners whichfirst atomise the fuel into small droplets, e.g. so-called“pressure-jet” burners, and wherein the combustion takes place on thesurface of the resulting small liquid droplets or takes place on theevaporated mixture of fuel and gas.

The Fischer-Tropsch derived fuel will comprise a Fischer-Tropsch productwhich may be any fraction of the middle distillate fuel range, which canbe isolated from the (hydrocracked) Fischer-Tropsch synthesis product.Typical fractions will boil in the naphtha, kerosene or gas oil range.Preferably a Fischer-Tropsch product boiling in the kerosene or gas oilrange is used because these fractions are easier to handle in forexample domestic environments. Such products will suitably comprise afraction larger than 90 wt % which boils between 160 and 400° C.,preferably to about 370° C. Examples of Fischer-Tropsch derived keroseneand gas oils are described in EP-A-583836, WO-A-9714768, WO-A-9714769,WO-A-011116, WO-A-011117, WO-A-0183406, WO-A-0183648, WO-A-0183647,WO-A-0183641, WO-A-0020535, WO-A-0020534, EP-A-1101813, U.S. Pat. No.5,766,274, U.S. Pat. No. 5,378,348, U.S. Pat. No. 5,888,376 and U.S.Pat. No. 6,204,426.

The Fischer-Tropsch derived product will suitably contain more than 80wt %, especially more than 90 wt % iso and normal paraffins and lessthan 1 wt % aromatics, the balance being naphthenics compounds. Thecontent of sulphur and nitrogen will be very low and normally below thedetection limits for such compounds. This low content of these elementsis due to the specific process wherein the Fischer-Tropsch reaction isperformed. The content of sulphur will therefore be below 5 ppm and thecontent of nitrogen will be below 1 ppm. As a result of the low contentsof aromatics and naphthenics compounds the density of theFischer-Tropsch product will be lower than the conventional mineralderived fuels. The density will be between 0.65 and 0.8 g/cm³ at 15° C.

The fuel used in the process of the present invention may also comprisefuel fractions other than the Fischer-Tropsch derived fuel product.Examples of such components may be the kerosene or gas oil fractions asobtained in traditional refinery processes, which upgrade crudepetroleum feedstock to useful products. Preferred non-Fischer-Tropschfuel fractions are the ultra low sulphur (e.g. less than 50 ppm sulphur)kerosene or diesel fractions, which are currently on the market.Optionally non-mineral oil based fuels, such as bio fuels, may also bepresent in the fuel composition. The content of the Fischer-Tropschderived product in the fuel will be preferably be above 40 wt %, morepreferably above 60 wt % and most preferably above 80 wt %. It should beunderstood that the content of such, currently less available,Fischer-Tropsch derived products will be optimised, wherein pricing ofthe total fuel will be balanced with the advantages of the presentinvention. For some applications fuels fully based on a Fischer-Tropschderived product plus optionally some additives may be advantageouslyused.

Evaporator burners are often provided with a flame detector. Mostdetectors, which are used today, detect a particular wavelengthassociated with the yellow colour of the flame. Applicants have nowfound that when a Fischer-Tropsch derived fuel is used the commonlyknown detectors fail to observe the resulting blue coloured flame. Forthis reason the evaporator burner is preferably provided with adetector, which can detect this blue flame. Examples of suitabledetectors are the detectors that are used in so-called blue flameburners. a flame detector is used. Examples of suitable detectors arethe UV sensors and IR sensors. A more preferred detector is theso-called ionisation sensor. An ionisation sensor is suitable to monitorburners with intermittent operation as well as continuous operation. Theprinciple of operation of the ionisation flame monitor is based on therectifying effect of a flame. If a flame is present, a current flowsbetween the burner an the ionisation electrode. This ionisation currentis evaluated by the flame monitor to determine if a flame is present. Insome prior art applications ionisation sensors could not be used incombination with a liquid fuel because deposits in the sensor led tofalse currents in the sensor. Because use of the Fischer-Tropsch derivedfuel, especially a fuel composition not containing a metal basedcombustion improver additive, results in less deposits ionisationsensors can be applied. This is an advantage because these sensors aremore readily available than the IR or UV sensors. Alternativelyadditives may be added to the Fischer-Tropsch derived fuel which resultin a flame which can be detected by the above standard detector.

The fuel may also comprise one or more of the following additives.Detergents, for example OMA 350 as obtained from Octel OY; stabilizers,for example Keropon ES 3500 as obtained from BASF Aktiengesellchaft, FOA528A as obtained from OCTEL OY; metal-deactivators, for example IRGAMET30 (as obtained from Speciality Chemicals Inc; (ashless) dispersants,for example as included in the FOA 528 A package as obtained from OctelOY; anti-oxidants; IRGANOX L57 as obtained from Speciality ChemicalsInc; cold flow improvers, for example Keroflux 3283 as obtained fromBASF Aktiengesellschaft, R433 or R474 as obtained from Infineum UK Ltd;anti-corrosion: Additin RC 4801 as obtained from Rhein Chemie GmbH,Kerocorr 3232 as obtained from BASF, SARKOSYL 0 as obtained from Ciba;re-odorants, for example Compensol as obtained from Haarman & Reiner;biocides, for example GROTA MAR 71 as obtained from Schuelke & Mayr;lubricity enhancers, for example OLI 9000 as obtained from Octel;dehazers, for example T-9318 from Petrolite; antistatic agents, forexample Stadis 450 from Octel; and foam reducers, for example TEGO 2079from Goldschmidt.

Applicants found that metal-based combustion improvers, which typicallyare added to the fuel composition used in the prior art method, can beleft out of the fuel. This is advantageous because as explained aboveionisation sensors may then be advantageously applied. Metal-basedcombustion improvers are for example ferrocene,methylcyclopentadienylmanganese-tricarbonyl (MMT).

The Fischer-Tropsch derived product is colourless and odourless. Forsafety reasons an odour marker, as for example applied in natural gasfor domestic consumption, may be present in the Fischer-Tropsch derivedproduct. Also a colour marker may be present to distinguish the fuelfrom other non-Fischer-Tropsch derived product.

The total content of the additives may be suitably between 0 and 1 wt %and preferably below 0.5 wt %.

The invention will now be illustrated with the following non-limitingexamples.

EXAMPLE 1

To a Jotul 709 Oven (as manufactured by Jotul ASA in Norway) aFischer-Tropsch derived kerosene having the properties as listed inTable 1 was operated in a period of 90 minutes. The feed rate was variedin time to simulate a practical domestic heating situation. Thevariation in feed rate was as listed in Table 2. TABLE 1 Fischer-TropschReference oil: kerosene Norway Kero Density (at 15° C.) 734.8 810Kinematic viscosity at 1.246 Not Measured 20° C. (mm²/s) Flash point (°C.) 43 Not Measured

TABLE 2 Feed rate Feed rate Time period (seconds) SMDS Kero (kg/h)Norway Kero 0 and 1800: middle load. 0.2 0.15  1800 and 3600: maximumload. 0.4 0.324 3600 and 5400: minimum load.  0.14 0.092

During the experiment the Smoke Number according to DIN 51402-1, thehydrocarbon content by means of photo ionization detector (FID) and thecarbon monoxide content by infrared spectroscopy in the exhaust gasesleaving the oven were measured. The results of these measurements arepresented in FIGS. 1 ^(a)-3 ^(a).

Comparative Experiment A

Example 1 was repeated except that commercial petroleum derived kerosenewas used of which the properties are listed in Table 1. The petroleumderived kerosene fuel used is currently used as fuel in evaporatingburner ovens in Norway.

The results of these measurements are also presented in FIGS. 1 ^(b)-3^(b).

As can be seen from FIGS. 1-3 is that the process according to thepresent invention results in a reduction of hydrocarbons and carbonmonoxide in the exhaust of the oven. This is very advantageous becausehealth, safety, environment and efficiency issues are improved.Emissions that are harmful to human health are reduced (soot andpotential carcinogenic potential). Also CO danger of suffocation in caseof leakages of exhaust gases into the room is reduced. A completecombustion, indicated by a lack of unburned hydrocarbons and low COemissions, also increase efficiency. Decreased hydrocarbon emissionsalso decrease the danger of chimney burns. Less soot deposits alsoprevent the formation of films on the heat exchanger surface, which candecrease the heat transfer and therefore the resulting efficiency.

1. A process to generate heat comprising burning a liquid fuel in anevaporator burner oven, wherein the liquid fuel comprises aFischer-Tropsch derived fuel.
 2. The process of claim 1, wherein theFischer-Tropsch derived fuel boils for more than 90 wt % between 160° C.and 400° C.
 3. The process of claim 2, wherein the Fischer-Tropschderived fuel boils for more than 90 wt % between 160° C. and 370° C. 4.The process of claim 1, wherein the Fischer-Tropsch derived fuelcomprises a Fischer-Tropsch product which contains more than 80 wt % ofiso and normal paraffins, less than 1 wt % aromatics, less than 5 ppmsulfur and less than 1 ppm nitrogen and wherein the density of theFischer-Tropsch product is between 0.65 and 0.8 g/cm³ at 15° C.
 5. Theprocess of claim 1, wherein the Fischer-Tropsch derived fuel comprisesmore than 80 wt % of a Fischer-Tropsch product.
 6. The process of claim5, wherein the Fischer-Tropsch derived fuel comprises a mineral oilfraction and/or a non-mineral oil fraction.
 7. The process one of claim1, wherein the Fischer-Tropsch derived fuel comprises one or moreadditives.
 8. The process of claim 7, wherein the Fischer-Tropschderived fuel comprises an odor marker.
 9. The process of claim 7,wherein the Fischer-Tropsch derived fuel comprises a color marker. 10.The process of claim 7, wherein an additive is present which changes thecolor of the flame such that is detectable by a yellow flame detector.11. The process of claim 1, wherein an ionization type flame detector isused to detect the flame of the evaporator burner and wherein the fueldoes not contain a metal-based combustion improver.